Replacement of aluminum surfaces and contacts in a semiconductor circuit by copper surfaces and contacts is attractive, for several reasons. Copper has nearly as high a relative electrical conductivity coefficient as silver (100 versus 106) and is much higher than the corresponding coefficient for gold (65), for aluminum (59) and for any other metal. The thermal conductivity coefficient for copper is also much higher than the corresponding thermal conductivity coefficient for aluminum. Copper has a higher melt temperature than aluminum (660° C. versus 1083° C.). Copper will form an oxidized surface when exposed to oxygen but will not form some of the disagreeable surface contaminants that aluminum forms under similar conditions. Use of a metal with higher electrical conductivity will allow use of smaller driving voltages, as low as 1.8 volts, and possibly lower, which in turn will not produce as much heat to be dissipated from the chip or other semiconductor device. Use of a metal with higher electrical conductivity and higher thermal conductivity will also allow choice of a greater range of lead frame materials for use with these devices.
However, generation and controllable deposition of a copper metal of a selected small thickness on a semiconductor surface or electrical contact is problematical, in part because such copper processes have not been developed as thoroughly as the corresponding aluminum processes. Cu has a modest electrode or reducing potential at T=25° C. (E0=0.32–0.34 volts), as compared to Ag, Au and Pt, for which E0 is of the order or 1 volt, and Al, for which E0 is about −1.7 volts. Cu has several oxidization states, as does Al.
What is needed is an approach for generation and controlled deposition of a selected thickness of copper metal on an exposed surface, such as a semiconductor material, of a workpiece, using pressures that may range from normal atmospheric pressure to several hundred psig, and using temperatures that may range from around −78° C. to around 100° C. Preferably, the approach should allow control of the rate of deposit of copper and the total thickness of copper deposited, through control of parameters such as ambient temperature, deposition time interval and electrodeposition voltage used.